The development of cancer involves somatic genetic changes leading to alterations in the control of cell proliferation Such alterations result in clonal cell accumulation at the site of tumor origin However, for a tumor to grow to a size sufficient to induce significant harm to the host through the processes of invasion or metastasis, additional alterations in the genetic control of cell death are often required. Members of the Bcl-2 family play a critical role in the genetic regulation of programmed cell death (apoptosis). The Bcl-2 family of proteins is comprised of two major subgroups. One subgroup, which includes the oncogenes Bcl-2 and Bcl-xL, is antiapoptotic while the other subgroup, which includes Bax and Bak, is proapoptotic when overexpressed. Although all family members have been shown to localize to intracellular membranes including the outer mitochondrial membrane, outer nuclear membrane, and endoplasmic reticulum during apoptosis, relatively little is known about the molecular mechanisms by which Bcl-2 proteins regulate cell survival. How, apoptotic activities of Bcl-2 family members are modulated by interacting proteins is also the subject of controversy. The major groups of Bcl-2 interacting proteins contain a BH3 domain through which they bind to Bcl-2 family members BH3 proteins have recently been proposed to be the triggers through which distinct signal transduction pathways initiate apoptosis. However, little is known about how the BH3 proteins are regulated because of their potent ability to induce apoptosis Recently, we have established mice and cell lines deficient in both Bax and Bak, the two ubiquitously-expressed proapoptotic, Bcl-2 family members. Bax-/- Bak -/- cells lack the ability to initiate apoptosis in response to a wide variety of apoptotic stimuli Using Bax-/- Bak -/- animals and cell lines, we seek to 1) explore the role of Bax and Bak in the regulation of mitochondrial membrane integrity, 2) examine whether selective re-expression of Bax or Bak to the endoplasmic reticulum can restore apoptosis in response to ER stress or other apoptotic stimuli, and 3) study the regulation of expression, post-translational modification, and intracellular distribution of proapoptotic BH3-containing proteins to further define their role in apoptosis. By carrying out these studies, we hope to better define the biochemical mechanisms that underlie apoptosis control by the Bcl-2 family. Through defining the molecular mechanisms by which Bcl-2 proteins contribute to the regulation of cell survival and death, we hope to contribute information that will lead to the development of new cancer treatments.